Thermal characterization of energy pile dynamics

Abstract

The heat transfer process in energy piles is strongly affected by the heat capacity of such foundation elements. This phenomenon is more pronounced for energy piles compared to borehole heat exchangers, because of the lower slenderness of the former compared to the latter, and involves axial thermal gradients. In literature, capacity effects of energy piles and their transient thermal performance have not been analysed in depth. Looking at such challenge, this paper investigates the dynamic thermal performance of energy piles at short-to-medium time scales. The work analyses the results of almost thirty 3D finite element simulations of an energy pile equipped with 3-U ducts by varying: (i) the velocity of the fluid circulating in the ducts, (ii) the slenderness ratio of the pile, (iii) the radial position of the ducts, and (iv) the boundary condition characterizing the uppermost surface of the model. Simulation results are analysed to identify for which times, geometries, and operative conditions the energy pile can be modelled with a 2D geometry, instead of a full 3D geometry. Our analysis highlights a limited relevance of the axial effects during the transient period in any tested configuration. These results are functional to the application of simplified analytical models and design criteria for energy piles.